A number of optical communication systems which have been put into practical use hitherto employ a binary modulation and demodulation technique using light intensity. Specifically, “0” and “1” of digital information are converted into ON and OFF of light intensity on a transmission side, the converted light is transmitted to an optical fiber, and the light propagating through the optical fiber undergoes photoelectric conversion on a reception side so as to be restored to original information. In recent years, with the explosively widespread use of the Internet, there has been a rapid increase of communication capacity which is a requirement of the optical communication system. A demand for a large communication capacity has been handled hitherto by increasing a speed of ON and OFF of light, that is, a modulation speed. However, the method of realizing a large capacity by increasing a modulation speed generally has the following problems.
If a modulation speed increases, there is a problem in that an allowable transmission distance limited by chromatic dispersion of an optical fiber is shortened. Generally, a transmission distance limited by the chromatic dispersion is shortened in proportion to the square of a bit rate. That is, if a bit rate doubles, a transmission distance limited by the chromatic dispersion becomes ¼. Similarly, if a modulation speed increases, there is also a problem in that an allowable transmission distance limited by polarization mode dispersion of an optical fiber is shortened. Generally, if a bit rate doubles, a transmission distance limited by the polarization mode dispersion becomes ½. When influence of the chromatic dispersion is described in detail, a transmission distance limited by the chromatic dispersion is 60 km if a normal dispersion fiber with a bit rate of 10 Gbps is used, but the distance is shortened to approximately 4 km if a system with a bit rate of 40 Gbps is used. Further, in a case of a 100 Gbps system of the next generation, a transmission distance limited by the chromatic dispersion becomes 0.6 km, and thus a trunkline optical communication system with a transmission distance of about 500 km cannot be realized in this state. In order to build an ultra-high speed trunkline optical communication system, a special optical fiber such as a so-called dispersion compensation fiber which has negative chromatic dispersion is currently installed in a relay or a transceiver so as to cancel out chromatic dispersion in a transmission path. This special fiber is expensive, and advanced design for determining an amount of dispersion compensation fiber installed in the transceiver or the optical relay is necessary, and both of them contribute to increase of the price of the optical communication system.
Therefore, in recent years, a study on an optical communication system using an OFDM technique has been attracting attention as an optical modulation and demodulation scheme for increasing a communication capacity. The OFDM technique is a technique in which amplitude and phase of each of a plurality of sine waves (called subcarriers) which are orthogonal to each other for one symbol time, that is, they have a frequency of integer multiples of a reciprocal of one symbol time are set to predetermined values, so as to carry information (modulate), and a carrier is modulated with a signal bundling these subcarriers and is transmitted. The OFDM technique is practically used for a Very high bit rate Digital Subscriber Line (VDSL) system which performs communication between a telephone service and households, a power-line communication system at home, or a digital terrestrial TV system. In addition, the technique is scheduled to be used for the next generation mobile phone system.
An optical OFDM communication system is a communication system which uses light as a carrier and employs the OFDM technique. In the OFDM technique, a plurality of subcarriers are used as described above, and, for example, a multilevel modulation method such as 4-QAM, 8-PSK, or 16-QAM can be used as a modulation method of each subcarrier, and thus one symbol time becomes much longer than a reciprocal of a bit rate. As a result, a transmission distance limited by the above-described chromatic dispersion or polarization mode dispersion becomes sufficiently longer than a transmission distance (for example, 500 km in a domestic trunkline system) expected in the optical communication system, and thereby the above-described dispersion compensation fiber becomes unnecessary. As a result, there is a possibility that a low cost optical communication system may be realized.
In an optical communication system using a direct detection reception method, unlike in a wireless communication system, a received optical current is proportional to the square of an absolute value of a field of light (on the other hand, in the wireless communication system, a current flowing through a reception antenna is proportional to a field). Due to this feature, a problem which is not present in the wireless OFDM communication occurs in the optical OFDM communication using the direct detection reception method. That is, since a received optical current is proportional to the square of an absolute value of an optical field, there is a problem in that a beat signal between subcarriers interferes with an original signal. This problem is hereinafter referred to as inter-subcarrier beat interference.